In a mobile communication system (for example, Personal Handy-Phone: PHS hereinafter), such a scheme is proposed in that a reception signal is extracted from a desired particular terminal by adaptive array processing in communications between a radio base apparatus (referred to as a base station hereinafter) and a mobile terminal apparatus (referred to as a terminal hereinafter), in particular, at a base station (see, for example, Toshinori Iinuma et al. “Adaptive Array Antenna PHS Base Station”, SANYO TECHNICAL REVIEW, Sanyo Electric Co., Ltd., published on May 1, 2000, Vol. 32, No. 1, pp. 80-88 (Non-Patent Document 1), and Yoshiharu Doi et al. “The Space Division Multiple Access PHS Base Station” SANYO TECHNICAL REVIEW, Sanyo Electric Co., Ltd., published on Dec. 10, 2001, Vol. 33, No. 3, pp. 93-101 (Non-Patent Document 2)).
Adaptive array processing refers to a process of calculating a reception weight vector made of a reception coefficient (weight) for each antenna, based on a signal received from a terminal at an array antenna formed of a plurality of antennas of a base station, and weighting the received signal with the reception weight vector to accurately extract a signal from a particular user terminal.
An adaptive array base station employing such adaptive array processing is provided with a reception weight vector calculator calculating a reception weight vector for each symbol of a reception signal. The reception weight vector calculator performs a process of allowing a reception weight vector to converge in such a manner as to decrease the square of an error between the complex multiplication sum (array output signal) of the reception signal and the calculated reception weight vector and a known reference signal corresponding to a particular user terminal in a known reference signal segment (weight estimation segment) provided at the head of each frame of the reception signal.
In the adaptive array processing, such convergence of a reception weight vector is adaptively carried out according to variations in the propagation path characteristic of a radio signal, so that an interference component and noise are eliminated from a reception signal thereby extracting the reception signal from a particular user terminal.
Such a reception weight vector calculator uses Minimum Mean Square Error (MMSE) method to calculate a means square error between an array output signal and a reference signal and calculate a reception weight such that the error is minimized. Minimum Mean Square Error method includes a plurality of control algorithms including RLS (Recursive Least Squares) algorithm, LMS (Least Mean Square) algorithm, SMI (Sample Matrix Inversion) algorithm, and the like for use in a mobile communication system.
RLS algorithm, LMS algorithm and SMI algorithm are well-known techniques in the field of adaptive array processing.
At the adaptive array base station, additionally performed as a part of adaptive array processing is a process of deciding transmission directivity to a user terminal and transmission power by weighting a transmission signal with a transmission weight vector formed by copying the reception weight vector calculated by the reception weight vector calculator.
A signal transmitted using the same array antenna as in reception is weighted with the same user terminal as in reception being targeted, so that the adaptive array base station outputs a transmission signal having high directivity targeting only the same user terminal. In other words, a transmission weight is formed such that a beam of a transmission radio wave is directed in the direction of a desired user terminal and a null of a transmission radio wave is directed in the direction of an interfering user terminal. Here, a beam means a direction in which radio waves emitted from each antenna intensify each other and a null means a direction in which radio waves emitted from each antenna weaken each other. Therefore, the interfering radiation characteristic, which is the amount of interference applied by the transmission radio wave from a base station to the other surrounding base station (cell), can be suppressed.
A method of controlling directivity of a transmission signal by a transmission weight vector formed by copying a reception weight vector calculated by MMSE-type algorithm (referred to as MMSE reception weight copy method hereinafter) is known (see, for example, Hideichi Sasaoka, “Mobile Communication”, Ohmsha, published on May 25, 1998, pp. 283-312 (Non-Patent Document 3)).
On the other hand, in stead of using a transmission weight vector formed by copying a reception weight vector as it is, a method of estimating a terminal arrival direction (a so-called reception response vector or simply a response vector) from a reception signal and estimating a transmission weight from the reception response vector (Zero-forcing method) is known (see, for example, Japanese Patent Laying-Open No. 2000-106539 (Patent Document 1)).
Here, a reception response vector represents information about the amplitude and phase of a signal from each terminal, of signal components from a terminal that is received at a base station. Such reception response vector of each terminal is estimated at a base station so that a propagation path characteristic in the radio area from each terminal to the base station, reception power and the like can be detected.
The method of estimating a reception response vector of a signal received from each terminal at a base station uses a technique of estimation by performing a complex multiplication of a reception signal (I signal and Q signal) received at each antenna of the base station and a re-modulation signal of known demodulation data corresponding to each terminal and obtaining the ensemble-average (time-average) of the result.
In the aforementioned Zero-forcing method, when a significant interference is measured, a reception response vector representing information of the arrival direction of a desired user terminal and an interfering user terminal is estimated, so that based on the estimated reception response vector, a transmission weight is formed such that a beam of a transmission radio wave is directed in the direction of the desired user terminal and a null of a transmission radio wave is directed in the direction of the interfering user terminal.
Accordingly, similarly to when a transmission weight is obtained by copying a reception weight, an adaptive array base station outputs a transmission signal having directivity targeting the desired user terminal, and in addition, the interfering radiation characteristic to other base stations (cells) can be suppressed.
As described above, at the adaptive array base station, an interference with a reception radio wave can be suppressed by the adaptive array processing and in addition, the interfering radiation characteristic to other base stations (cells) can be improved by transmission directivity control based on the above-noted MMSE reception weight copy method and Zero-forcing method, whereby the frequency usage efficiency can be improved in the mobile communication system as a whole.
However, although the transmission directivity control such as MMSE reception weight copy method and Zero-forcing method can be used to suppress the interfering radiation characteristic, which is the amount of interference applied by a transmission radio wave of the base station to other surrounding base stations (cells), the optimum transmission does not always result in view of the communication quality for the desired user terminal itself.
For example, in the adaptive array reception using the above-noted MMSE-type algorithm, a reception weight is generated such that a signal-to-noise ratio is maximized. Therefore, for example when a power difference is big in a desired wave component in a reception signal at each antenna included in an array antenna, a reception weight for each antenna widely varies.
Then, if the widely-varied reception weight is applied as it is as a transmission weight, the transmission power of an antenna of a part of the array antenna significantly reduces, resulting in that radio waves can actually be transmitted only from the remaining part of antennas of the array antenna. In such a case, the transmission power of the entire array antenna significantly reduces.
More specifically, although directivity for a desired user terminal is formed, reception power in the desired user terminal may be reduced when a communication environment considerably changes such that a new interference wave occurs before a reception weight is generated and reflected in a transmission signal. In other words, reception power at a desired user terminal cannot always be maximized.
Therefore, in MMSE reception weight copy method and Zero-forcing method, the transmission power level at a base station is insufficient, so that the communication quality at the desired user terminal may be degraded.
On the other hand, a technique of in-phase combination (referred to as the in-phase combination maximum transmission method) is known in which such a transmission weight is calculated that allows a reception power level at a desired user terminal to be maximized (see, for example, Japanese Patent Laying-Open No. 11-274976 (Patent Document 2) and Japanese Patent Laying-Open No. 2000-151487 (Patent Document 3)).
The basic principle of the in-phase combination will be described briefly. In the in-phase combination maximum transmission method, a phase of a transmission weight is determined based on a phase of a reception response vector of each antenna such that a reception power at a terminal in the terminal arrival direction (reception response vector) estimated by the adaptive array processing is maximum. More specifically, a transmission weight is generated such that the multiplication result in each antenna is only a real number by multiplying a reception response vector for each antenna included in an array antenna by a transmission weight corresponding to each antenna. Accordingly, the desired user terminal can receive each transmission signal in phase from the array antenna of the base station, and the reception power at the desired user terminal can be maximized. It is noted that the amplitude of the reception response vector becomes the amplitude of the transmission weight as it is.
As described above, since the schemes of transmission directivity control at the adaptive array base station are different in interfering radiation characteristics and reception power characteristics at a desired user terminal, there has conventionally been proposed a scheme of selecting each transmission directivity control scheme according to a communication environment rather than supporting only one of the transmission directivity control schemes (see, for example, Japanese Patent Laying-Open No. 2004-153527 (Patent Document 4)).
On the other hand, PHS additionally employs a modulation scheme with a number of levels larger than the initially employed π/4 shift QPSK (Quadrature Phase Shift Keying) modulation scheme in order to support data communications which require a bulk transmission as compared with the conventional voice communications. As an example of such a multilevel modulation scheme, 16QAM (Quadrature Amplitude Modulation) modulation scheme or the like is known.
In the modulation scheme with a large number of levels such as 16QAM modulation scheme, an interval between symbols is narrow and symbols are densely arranged, so that it is likely that a symbol is erroneously recognized in the case of a bad communication environment, and a reception error tends to occur, although the data rate is high as compared with π/4 shift QPSK modulation scheme.
Moreover, BPSK modulation scheme is advantageous in that a reception error is less likely to occur since the noise immunity is high, although the data rate is low as compared with π/4 shift QPSK modulation scheme and 16QAM modulation scheme.
In this way, since the modulation schemes are different in data rate and noise immunity, an adaptive modulation scheme has conventionally been proposed in which a terminal and a base station select each modulation scheme according to a communication environment rather than supporting only one of the modulation schemes (see, for example, Japanese Patent Laying-Open No. 2003-298670 (Patent Document 5)).    Patent Document 1: Japanese Patent Laying-Open No. 2000-106539    Patent Document 2: Japanese Patent Laying-Open No. 11-274976    Patent Document 3: Japanese Patent Laying-Open No. 2000-151487    Patent Document 4: Japanese Patent Laying-Open No. 2004-153527    Patent Document 5: Japanese Patent Laying-Open No. 2003-298670    Non-Patent Document 1: Toshinori Iinuma et al. “Adaptive Array Antenna PHS Base Station”, SANYO TECHNICAL REVIEW, Sanyo Electric Co., Ltd., published on May 1, 2000, Vol. 32, No. 1, pp. 80-88    Non-Patent Document 2: Yoshiharu Doi et al. “The Space Division Multiple Access PHS Base Station” SANYO TECHNICAL REVIEW, Sanyo Electric Co., Ltd., published on Dec. 10, 2001, Vol. 33, No. 3, pp. 93-101    Non-Patent Document 3: Hideichi Sasaoka, “Mobile Communication”, Ohmsha, published on May 25, 1998, pp. 283-312